1. Field of the Invention
This invention relates to a solenoid valve whose sliding plunger makes a valve spool move.
2. Discussion of the Background
A conventional solenoid valve may be installed into an electronically controlled automatic transmission. The solenoid valve is inserted into a valve body formed in an oil pan of the automatic transmission. Japanese patent laid open 01-242884 is prior art disclosing such a solenoid valve. The prior art solenoid valve includes a solenoid portion and a valve portion. In the solenoid portion, there is a stator core which is coaxially constructed by a core and a yoke in series. A bore is formed at the axial center of the stator core, and a plunger is slidably fitted into the bore. The valve portion coaxially adjoins the solenoid portion and is constructed by a valve sleeve and a spool slidably fitted into the valve sleeve. The plunger is axially moved against a spring by an electromagnetic coil through the magnetized stator core and moves the spool. The solenoid valve has a cover to separate the solenoid portion from the valve portion. There is a space surrounded by the bore, one end of the plunger and the cover. The volume in the space changes due to the sliding of the plunger. In a conventional solenoid valve, the space is connected with the outside via an inlet/outlet passage to absorb the volume changes, so that contaminants in the oil may be sucked into the space from the outside.
In the prior art, since the plunger slides in the bore, the contaminants may become lodged between the bore and the plunger, whereby the plunger may lock. To resolve such a problem, the plunger of the prior art is constructed by a magnetic hollow slide and one end of a shaft is inserted into the slide. The other end of the shaft projects from the slide and is inserted into a concave portion of the spool. Therefore, there reliably exists a gap between the slide and the bore of the stator core. However, the wider gap increases the magnetic reluctance in the magnetic circuit, which reduces magnetization of the stator core and the slide. Therefore the magnetic attraction force to the plunger is reduced and the valve characteristics become unstable, or the electromagnetic coil must become larger to obtain suitable valve characteristics.
To resolve the above problem, in Japanese laid open 2002-310322 a diaphragm is disposed at one end of the spool next to the plunger in order to prevent the contaminants from being sucked in. In addition, a cover is disposed at the end of the plunger opposite to the spool and a breathing port is formed in the cover to connect the inside/outside of the solenoid portion. In the second prior art, however, the diaphragm requires a rubber material which has oil resistance and soft elasticity even in a high/low temperature environment, whereby its product costs are higher.